Optical pulse contrast improvement using nonlinear conversion

ABSTRACT

A laser radar (LADAR) system includes a laser transmitter configured to emit laser pulses at a first wavelength, a non-linear converter configured to convert the laser pulses to a second wavelength prior to spectral filtering of amplified spontaneous emission (ASE) that is emitted from the laser transmitter in a spectrum concentrated around the first wavelength, and a spectral filter configured to substantially filter the ASE and allow the laser pulses at the second wavelength to pass.

TECHNICAL FIELD

The present disclosure is directed in general to LADAR systems and morespecifically to the use of nonlinear conversion techniques to improveoptical pulse contrast in LADAR systems.

BACKGROUND OF THE DISCLOSURE

A LADAR (Laser Detection and Ranging) system is a laser-based radarsystem that has many applications, including those in the defenseindustry. Like conventional radar systems, LADAR systems transmit,receive, and detect electromagnetic waves that reflect from a target.Instead of operating in millimeter and microwave wavelengths, likeconventional radar, LADAR systems operate at laser wavelengths, such asin the micrometer and sub-micrometer bands.

SUMMARY OF THE DISCLOSURE

To address one or more deficiencies of the prior art, one embodimentdescribed in this disclosure provides a laser radar (LADAR) system. TheLADAR system includes a laser transmitter configured to emit laserpulses at a first wavelength, a non-linear converter configured toconvert the laser pulses to a second wavelength prior to spectralfiltering of amplified spontaneous emission (ASE) that is emitted fromthe laser transmitter in a spectrum concentrated around the firstwavelength, and a spectral filter configured to substantially filter theASE and allow the laser pulses at the second wavelength to pass.

Another embodiment in this disclosure provides a method for use in aLADAR system. The method includes emitting, at a laser transmitter,laser pulses at a first wavelength and ASE in a spectrum concentratedaround the first wavelength; converting, at a non-linear converter, thelaser pulses to a second wavelength prior to spectral filtering of theASE; and substantially filtering, at a spectral filter, the ASE andallowing the laser pulses at the second wavelength to pass.

A further embodiment in this disclosure provides a Geiger mode LADARsystem. The Geiger mode LADAR system includes a Geiger mode lasertransmitter configured to emit laser pulses at a first wavelength, anon-linear converter configured to convert the laser pulses to a secondwavelength prior to spectral filtering of ASE that is emitted from thelaser transmitter in a spectrum concentrated around the firstwavelength, and a spectral filter disposed in a transmit path of theGeiger mode LADAR system, where the spectral filter is configured tosubstantially filter the ASE and allow the laser pulses at the secondwavelength to pass.

Although specific advantages have been enumerated below, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the following figuresand description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an example portion of a Geiger mode LADAR system,according to this disclosure;

FIG. 2 illustrates an example non-linear converter (NLC) for use in aLADAR system, according to this disclosure;

FIG. 3 illustrates a graphical representation of an example operation ofthe NLC of FIG. 2, according to this disclosure;

FIG. 4 illustrates a graphical representation of an ASE (amplifiedspontaneous emission) power spectral density of a laser amplifier foruse in an example LADAR system, according to this disclosure; and

FIG. 5 illustrates an example dual Geiger mode and coherent mode LADARsystem, according to this disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 5, described below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any type of suitably arranged device or system.

A Geiger mode avalanche photodiode (GMAPD) LADAR system is a type ofLADAR system that is used for 3D imaging of objects at long range byprecisely measuring a time of flight for short duration laser pulses.GMAPDs are single photon sensitive, with a dead time following eachdetection event. Amplified spontaneous emission (ASE) from pulsed lasertransmitters can be problematic in GMAPD detection systems, because theASE is a continuous source of photons that can blind the detector,preventing the detection of a return pulse, especially in sharedaperture LADAR systems. The ASE can be filtered, for example, by angle,wavelength, and polarization. However, in-band and in-beam ASE may bedifficult to filter, and can reach the detection system before the laserpulse arrives. This is problematic due to the dead time associated withGMAPD detectors after a detection event. The ASE photons that arrivebefore the return laser pulse can blind the detector and cause a falsetime of flight measurement.

To address these issues, embodiments of this disclosure eliminate thein-band and in-beam components of the ASE by using nonlinear opticalfrequency conversion to shift the wavelength of the transmitted laserpulse. Because the conversion efficiency of the nonlinear process isintensity dependent, the short duration laser pulse will convertefficiently, while the low intensity ASE will be unaffected. Thewavelength shifted pulse may then be spectrally filtered from the ASE.

The embodiments disclosed herein provide an intensity-dependentwavelength shift to allow separation of laser pulses from ASE throughspectral filtering. The use of the intensity-dependent wavelength shiftat the transmit end of a Geiger mode LADAR system combined with aspectral filter at the receive end serves to block ASE from reaching theGeiger mode detector. In some embodiments, the intensity-dependentwavelength shift induced at the transmit end is used to spectrallydistinguish between returns intended for coherent or Geiger modedetectors at the receive end of the LADAR system. Also, becausewavelengths that are advantageous for atmospheric transmission do notalways coincide with preferred operating wavelengths of lasertransmitters, the disclosed embodiments provide an added benefit byallowing the laser transmitter wavelength to be shifted to a betteratmospheric window.

It will be understood that embodiments of this disclosure may includeany one, more than one, or all of the features described herein. Inaddition, embodiments of this disclosure may additionally oralternatively include other features not listed herein.

FIG. 1 illustrates example components of a transmit path of a Geigermode LADAR system, according to this disclosure. The embodiment of theLADAR transmit path 100 illustrated in FIG. 1 is for illustration only.The transmit path components 100 illustrated in FIG. 1 may representonly a portion of a complete Geiger mode LADAR system which may alsoinclude a receiver (sensor) path (not shown in FIG. 1). Otherembodiments could be used without departing from the scope of thisdisclosure.

In a LADAR system that utilizes a transmit path such as the LADARtransmit path 100, short laser pulses are generated by a lasertransmitter. The laser pulses are transmitted over a distance, hit atarget, and then echo back, where they are received by a receiver of theLADAR system, such as a Geiger mode receiver. A Geiger mode receiver isa photon counter and is sensitive to background light at the singlephoton level, including the ASE generated by the LADAR transmitteritself.

The ASE is low power, especially compared to the laser pulse (e.g., ASEmay have a power on the order of 0.1 milliwatts, which is low comparedto an outgoing laser pulse that may have a peak power on the order of 10megawatts). A laser signal pulse returning from a target echo is of lowintensity and may be at a level comparable to or below that of the ASE,which is typically emitted from the laser transmitter even during timeswhen there is no outgoing laser pulse. The ASE emitted by the laser canreach the Geiger-mode sensor through reflection or scattering within theLADAR system. This reflected or scattered ASE can blind the sensor,thereby preventing the detection of a return pulse.

To resolve this issue, the output of the laser transmitter is passedthrough a non-linear conversion device, which shifts the wavelength ofthe laser pulse out of the spectral band of the ASE. This allows aspectral filter to block the ASE but permit the wavelength-shifted laserpulse to pass through to the receiver.

As shown in FIG. 1, the LADAR transmit path 100 includes a Geiger modelaser transmitter 110, a non-linear converter (NLC) 120, and a spectralfilter 130. The LADAR transmit path 100 may include other components;however, descriptions of such components are not necessary and areomitted herein for clarity. As used herein, a Geiger mode lasertransmitter refers to a laser transmitter that emits optical pulses andis used in conjunction with a Geiger mode sensor in a LADAR system.

The laser transmitter 110 may comprise any one of a number of suitablelasers or laser amplifiers. For example, the laser transmitter 110 mayinclude a Nd:YAG (Neodymium-doped yttrium aluminum garnet (YAG))amplifier, a Yb:YAG (Ytterbium-doped YAG) amplifier, a Ho:YAG(Holmium-doped YAG) amplifier, an Er:YAG (Erbium YAG) amplifier, anEr:glass amplifier, a Tm:YAG (Thulium-doped YAG) amplifier, variationsof these amplifiers which are co-doped with a sensitizer ion (e.g.,Er,Yb:glass), or any other suitable laser source or amplifier. The lasertransmitter 110 emits short laser pulses 112 at a wavelength λ_(L). Thewavelength λ_(L) is dependent on the type of laser used in thetransmitter 110. The laser transmitter 110 also emits ASE 115 in aspectrum surrounding the same wavelength λ_(L) as the laser pulses 112.

The NLC 120 is a non-linear converter, also referred to as a non-linearwavelength shifter. The NLC 120 may be comprised of a crystallinematerial. In some embodiments, the NLC 120 is an optical parametricamplifier (OPA), as described in greater detail below. The NLC receivesthe laser pulses 112 and the ASE 115 emitted from the laser transmitter110. The NLC 120 converts (or shifts) the laser pulses 112 at wavelengthλ_(L) to pulses 122 at a different wavelength λ_(C). The conversionefficiency of the NLC 120 is dependent on the intensity of the inputsignal, as well as a phase matching condition. Thus, the high intensitylaser pulses 112 are subject to the wavelength shift, but the lowintensity, in-band, in-beam ASE 115 is substantially unaffected, asdescribed in greater detail below. Accordingly, the NLC 120 does notconvert the wavelength of the ASE 115.

The wavelength shift is configurable and may be selected based on thebandwidth of the ASE 115 from the laser transmitter 110. That is, inorder to adequately filter out the ASE 115, the wavelength shiftproduced by the NLC 120 should be to a wavelength λ_(C) that is outsideall (or substantially all) of the bandwidth of the ASE 115. This ASEbandwidth varies with the type of laser transmitter used. For example,the bandwidth of the ASE 115 could range from nanometers for one micronemission from Nd:YAG and Yb:YAG materials, to tens of nanometers for twomicron emission from Ho:YAG material. Ideally, the shifting of the laserpulse wavelength is to a spectral region that is devoid of laser ASE.

The spectral filter 130 serves to separate the converted laser pulses122 at the wavelength λ_(C) from the ASE 115 concentrated around thewavelength λ_(L). In some embodiments, the spectral filter 130 islocated in the receiver (sensor) path of the LADAR system. That is, thespectral filter 130 directs signals that have already been transmittedto the target object and have echoed back to the LADAR system to theGeiger mode sensor (detector). The spectral filter 130 directs theconverted laser pulses 122 at the wavelength λ_(C) to the Geiger modesensor and filters out all, or a substantial portion, of the ASE 115around the wavelength λ_(L), thereby leaving only the converted laserpulses 132 at the wavelength λ_(C) to be sensed (detected) by the Geigermode receiver of the LADAR system. Typically, ASE photons in thereceiver path are those that are scattered from optics or componentswhich are common to the transmit and receiver paths.

FIG. 2 illustrates an example non-linear converter (NLC) for use in aLADAR system, according to this disclosure. The embodiment of the NLC200 illustrated in FIG. 2 is for illustration only. Other embodimentscould be used without departing from the scope of this disclosure. Insome embodiments, the NLC 200 may represent the NLC 120 of FIG. 1.

As shown in FIG. 2, the NLC 200 receives laser pulses 202 at aparticular wavelength (such as 2091 nm) and converts (or shifts) thelaser pulses 202 to converted signal pulses 210 at a differentwavelength (such as 2135 nm). The laser pulses 202 may represent thelaser pulses 112 at the wavelength λ_(L), and the converted signalpulses 210 may represent the converted pulses 122 at the wavelengthλ_(C), as shown in FIG. 1.

In some embodiments, the NLC 200 is an optical parametric amplifier(OPA). An optical parametric amplifier typically comprises a crystallinematerial and employs a three-wave mixing process in which a very strongsignal at one wavelength (e.g., the laser pulses 202) amplifies lowlevel light at the desired wavelength. Using a signal seed 205 at thedesired wavelength, the OPA 200 uses an intensity dependent nonlinearoptical process to convert (or shift) the laser pulses 202 at the 2091nm wavelength to the converted signal pulses 210 at the 2135 nmwavelength. Signal gain is achieved at the high peak power of the laserpulse. Therefore, the OPA 200 does not shift the wavelength of the ASEphotons in the absence of the high peak power laser pulses 202. Thesignal seed 205 is pulsed, with its timing synchronized to the timing ofthe laser input pulses 202. The signal seed 205 is turned off betweenlaser pulses. In some embodiments, the signal seed 205 is generatedusing a semiconductor laser which can be rapidly turned off and on. Asemiconductor laser does not store energy and therefore does notgenerate ASE around the signal wavelength when it is turned off (betweenpulses 202).

Because the OPA 200 is a photon energy conserving device, the OPA 200also generates an idler signal 212 as a byproduct. In the embodimentdescribed here, the frequency of the idler signal 212 is in theterahertz range. The OPA 200 also allows a small portion 215 of thelaser pulses 202 to pass through unconverted.

In another embodiment, the NLC 200 may convert the laser pulsewavelength 202 using Raman shifting. A Raman shifting NLC could alsocomprise a crystalline material, but may comprise other materials. InStokes shifted Stimulated Raman scattering, the frequency of the laserpulses is reduced, and the energy difference between the input pulsesand the output pulses goes into the medium as heat (phonons). Thus, in aStokes shifted Stimulated Raman scattering NLC, the photons of the idlersignal 212 are replaced by phonons.

In some embodiments, it is preferable to use a wavelength shiftingprocess in the NLC 200 that results in a minimum change in wavelengthcompatible with the ASE bandwidth. This minimizes the loss of energyfrom the laser pulse 202 due to the quantum defect associated with theconversion process. This loss of energy is manifest as the idler photons212 in parametric conversion or phonons in a Raman shift conversion.

Although the NLC 200 is described and shown as operating using 2091 nmand 2135 nm wavelengths, it will be understood that this disclosure isnot limited thereto, and other suitable laser wavelengths are possible.

FIG. 3 illustrates a graphical representation of an example operation ofthe NLC 200 of FIG. 2, according to this disclosure. The embodiment ofthe operation illustrated in FIG. 3 is for illustration only. Otherembodiments could be used without departing from the scope of thisdisclosure.

As shown in FIG. 3, the laser signal input to the NLC 200 includes thelaser pulses 202 and the laser ASE 203. The laser pulses 202 and thelaser ASE 203 are concentrated in a spectral region around the laserwavelength λ_(L) (e.g., 2091 nm). However, the laser ASE 203 can bespread across the whole gain spectrum of the laser amplifier. Theportion of the laser ASE spectrum 203 that is in-band with the laserpulses 202 is indicated by reference numeral 203 a.

In operation, the NLC 200 shifts the wavelength λ_(L) of the input laserpulses 202 to a converted wavelength λ_(C) (e.g., 2135 nm). As describedabove, the wavelength shift generated by the NLC 200 is preferably largeenough such that the wavelength λ_(C) of the converted laser pulses 210is substantially or completely outside of the ASE spectrum 203. Aspectral filter (e.g., the spectral filter 130) filters out the laserASE 203 and allows the converted laser pulses 210 to pass, therebysignificantly improving the pulse contrast.

The conversion process of the NLC 200 may result in a small loss ofpulse intensity. However, the trade-off between pulse power loss and thebenefit of isolating the desired laser pulses from the undesired ASE isacceptable.

FIG. 4 illustrates a graphical representation of an ASE power spectraldensity of a laser amplifier for use in an example LADAR system,according to this disclosure. The embodiment illustrated in FIG. 4 isfor illustration only. Other embodiments could be used without departingfrom the scope of this disclosure.

As shown in FIG. 4, a chart 400 illustrates the ASE power spectraldensity (PSD) of a single-pass Ho:YAG laser amplifier. Such an amplifiermay be used in a laser transmitter in a LADAR system (e.g., the lasertransmitter 110 in the LADAR system 100). The spectrum of the laser ASEincludes wavelengths from approximately 1800 nm to approximately 2150nm, with a peak gain at 2091 nm.

A chart 402 shows an enlarged view of a portion of the chart 400indicated by the box. As shown in the enlarged chart 402, the powerspectral density of the ASE at 2135 nm is approximately oneone-hundredth of the power spectral density of the ASE at the 2091 nmpeak. Stated another way, the power spectral density of the ASE at 2135nm is at least 20 dB less than the power spectral density at 2091 nm.Thus, if a non-linear converter (e.g., the NLC 200) shifts thewavelength of the laser pulses from 2091 nm to 2135 nm, the in-band ASEat the converted wavelength (2135 nm) is reduced by 20 dB or more ascompared to the in-band ASE at the laser wavelength (2091 nm). This is asignificant reduction for a relatively modest wavelength shift.

FIG. 5 illustrates an example dual Geiger mode and coherent mode LADARsystem, according to this disclosure. In a dual mode LADAR system, twosensors are pointed at the same target and some means of directing thereturn to the proper sensor is needed. The dual mode LADAR system 500provides such a means. The embodiment of the LADAR system 500illustrated in FIG. 5 is for illustration only. Other embodiments couldbe used without departing from the scope of this disclosure. In someembodiments, the LADAR system 500 may represent or include the LADARtransmit path 100 shown in FIG. 1.

In the system 500, coherent and Geiger mode transmit beams are generatedat the same wavelength. The system 500 can shift the wavelength of thehigh-intensity pulses of the Geiger mode transmit beam, but leave thelow-intensity coherent-mode wavelength unchanged. The Geiger mode andcoherent mode return signals can then be spectrally distinguished at theLADAR receiver. This is particularly useful if separate detector arraysare employed for Geiger and coherent modes of operation. In that case,the received light can be directed to either the Geiger mode or coherentmode arrays via a dichroic beamsplitter, as described below.

As shown in FIG. 5, a stable seed laser 501 generates a laser output.Depending on the mode of operation, the output from the stable seedlaser 501 is processed by a Geiger mode pulse modulator (i.e., anintensity modulator) 503 to generate short Geiger mode pulses, or istransmitted to a coherent modulator (i.e., a phase modulator) 505, whichchirps the frequency of the laser output. The laser output is thenreceived at one or more pre-amp stages 507, which are typically fiberamplifiers, and then transmitted to a power amplifier 509, whichamplifies the pulsed laser output produced in Geiger mode operation, orthe chirped CW laser output produced in coherent mode operation,depending on which mode of operation is selected.

The amplified light from the power amplifier then enters the nonlinearconverter (or wavelength shifter) 511. In some embodiments, the NLC 511may be the same as or similar to the NLC 120 of FIG. 1 or the NLC 200 ofFIG. 2. In the NLC 511, the high peak intensity pulses for Geiger modeoperation are shifted in wavelength, such as described with respect toFIG. 2. However, the low intensity continuous wave (CW) laser outputproduced in the coherent mode of operation and the low intensity ASEproduced in the Geiger mode of operation are not converted or shifted inthe NLC 511, and remain at the same wavelength. In an OPA implementationof the NLC 511, the NLC 511 receives input from a seed diode 510, whichis turned off between pulses. The seed diode 510 provides the seed inputat the shifted wavelength, such as described in FIG. 2.

The light from the NLC 511 passes through beam shaping optics 513 and anaperture 515, where it is then directed to the target. Once the lightechoes off the target, it is received again at the aperture 515 andtransmitted to a spectral beamsplitter 517 in the receive path. Thespectral beamsplitter 517 is a dichroic beamsplitter configured toreceive laser signals from both modes of operation (i.e., Geiger modeand coherent mode). Based on the wavelength of the received lasersignals, the spectral beamsplitter 517 directs the received lasersignals along a Geiger mode receive path to a Geiger mode sensor, oralong a coherent mode path to a coherent sensor. For example, thespectral beamsplitter 517 may reflect the spectrum where the coherentlaser operates, and transmit the spectrum where the Geiger mode laseroperates.

For example, if the received laser signals comprise wavelength shiftedGeiger mode pulses, the beamsplitter 517 passes the laser signals alongthe Geiger mode receive path to a Geiger mode avalanche photodiode (APD)focal plane array (FPA) 519. Alternatively, if the received lasersignals comprise CW laser output or ASE scattered back from optics, thebeam splitter 517 passes the laser signals along the coherent modereceive path to a linear mode APD and FPA 521. The linear mode APD FPA521 performance is not degraded by ASE.

Thus, wavelength shifting of laser pulses in the NLC 511 enables passiveswitching of the return signal between coherent and Geiger mode sensorsusing spectral separation at the beamsplitter 517. Therefore, noprogramming or hardware is required to actively switch between the FPAsin the receive path.

Although FIGS. 1 through 5 illustrate example embodiments of thisdisclosure, various changes may be made to FIGS. 1 through 5. Forexample, the various components shown in each of FIGS. 1 through 5 maybe incorporated in other figures without departing from the scope ofthis disclosure. Further, the makeup and arrangement of the variouscomponents is for illustration only. Components could be added, omitted,combined, arranged in a different location or order, or placed in anyother configuration according to particular needs.

It may be advantageous to set forth definitions of certain words andphrases used throughout this patent document. The term “couple” and itsderivatives refer to any direct or indirect communication between two ormore elements, whether or not those elements are in physical contactwith one another. The terms “include” and “comprise,” as well asderivatives thereof, mean inclusion without limitation. The term “or” isinclusive, meaning and/or. The phrase “associated with,” as well asderivatives thereof, means to include, be included within, interconnectwith, contain, be contained within, connect to or with, couple to orwith, be communicable with, cooperate with, interleave, juxtapose, beproximate to, be bound to or with, have, have a property of, have arelationship to or with, or the like. The phrase “at least one of,” whenused with a list of items, means that different combinations of one ormore of the listed items may be used, and only one item in the list maybe needed. For example, “at least one of: A, B, and C” includes any ofthe following combinations: A, B, C, A and B, A and C, B and C, and Aand B and C.

Modifications, additions, or omissions may be made to the systems,apparatuses, and methods described herein without departing from thescope of the invention. The components of the systems and apparatusesmay be integrated or separated. Moreover, the operations of the systemsand apparatuses may be performed by more, fewer, or other components.The methods may include more, fewer, or other steps. Additionally, stepsmay be performed in any suitable order. As used in this document, “each”refers to each member of a set or each member of a subset of a set.

Various functions described herein can be implemented or supported byone or more computer programs, each of which is formed from computerreadable program code and embodied in a computer readable medium. Theterms “application” and “program” refer to one or more computerprograms, software components, sets of instructions, procedures,functions, objects, classes, instances, related data, or a portionthereof adapted for implementation in a suitable computer readableprogram code. The phrase “computer readable program code” includes anytype of computer code, including source code, object code, andexecutable code. The phrase “computer readable medium” includes any typeof medium capable of being accessed by a computer, such as read onlymemory (ROM), random access memory (RAM), a hard disk drive, a compactdisc (CD), a digital video disc (DVD), or any other type of memory. A“non-transitory” computer readable medium excludes wired, wireless,optical, or other communication links that transport transitoryelectrical or other signals. A non-transitory computer readable mediumincludes media where data can be permanently stored and media where datacan be stored and later overwritten, such as a rewritable optical discor an erasable memory device.

To aid the Patent Office, and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists onthe date of filing hereof unless the words “means for” or “step for” areexplicitly used in the particular claim.

1. A laser radar (LADAR) system, the LADAR system comprising: a lasertransmitter configured to emit laser pulses at a first wavelength; anon-linear converter configured to convert the laser pulses to a secondwavelength prior to spectral filtering of amplified spontaneous emission(ASE) that is emitted from the laser transmitter in a spectrumconcentrated around the first wavelength; and a spectral filterconfigured to substantially filter the ASE and allow the laser pulses atthe second wavelength to pass.
 2. The LADAR system of claim 1, whereinthe second wavelength is outside of at least a substantial portion ofthe ASE spectrum.
 3. The LADAR system of claim 1, wherein the non-linearconverter employs an intensity-dependent conversion process thatconverts the laser pulses to the second wavelength but does notsubstantially affect the ASE.
 4. The LADAR system of claim 1, whereinthe non-linear converter comprises an optical parametric amplifierconfigured to receive a signal seed at the second wavelength and convertthe laser pulses to the second wavelength by amplifying the signal seed.5. The LADAR system of claim 1, wherein the non-linear convertercomprises a Raman shifting wavelength converter.
 6. The LADAR system ofclaim 1, wherein the laser transmitter comprises a Geiger mode lasertransmitter.
 7. The LADAR system of claim 1, wherein the lasertransmitter comprises one of a Nd:YAG (Neodymium-doped yttrium aluminumgarnet (YAG)) amplifier, a Yb:YAG (Ytterbium-doped YAG) amplifier, aTm:YAG (Thulium-doped YAG) amplifier, a Ho:YAG (Holmium-doped YAG)amplifier, an Er:YAG (Erbium YAG) amplifier, an Er:glass amplifier, oran Er,Yb:glass amplifier.
 8. The LADAR system of claim 1, wherein theLADAR system is a dual Geiger mode and coherent mode LADAR system, theLADAR system further comprising: a Geiger mode pulse modulatorconfigured to generate Geiger mode laser pulses; a coherent modulatorconfigured to generate a coherent LADAR waveform laser signal at thefirst wavelength; and a passive beamsplitter in a receive path of theLADAR system, the passive beamsplitter configured to separate the laserpulses at the second wavelength from the continuous wave laser signal atthe second wavelength.
 9. A method for use in a laser radar (LADAR)system, the method comprising: emitting, at a laser transmitter, laserpulses at a first wavelength and amplified spontaneous emission (ASE) ina spectrum concentrated around the first wavelength; converting, at anon-linear converter, the laser pulses to a second wavelength prior tospectral filtering of the ASE; and substantially filtering, at aspectral filter, the ASE and allowing the laser pulses at the secondwavelength to pass.
 10. The method of claim 9, wherein the secondwavelength is outside of at least a substantial portion of the ASEspectrum.
 11. The method of claim 9, wherein the wavelength conversioncomprises an intensity-dependent conversion process that converts thelaser pulses to the second wavelength but does not substantially affectthe ASE.
 12. The method of claim 9, wherein the non-linear convertercomprises an optical parametric amplifier, and wherein converting thelaser pulses further comprises: receiving, at the optical parametricamplifier, a signal seed at the second wavelength and converting thelaser pulses to the second wavelength by amplifying the signal seed. 13.The method of claim 9, wherein the non-linear converter comprises aRaman shifting wavelength converter.
 14. The method of claim 9, whereinthe laser transmitter comprises a Geiger mode laser transmitter.
 15. Themethod of claim 9, wherein the laser transmitter comprises one of aNd:YAG (Neodymium-doped yttrium aluminum garnet (YAG)) amplifier, aYb:YAG (Ytterbium-doped YAG) amplifier, a Tm:YAG (Thulium-doped YAG)amplifier, a Ho:YAG (Holmium-doped YAG) amplifier, an Er:YAG (ErbiumYAG) amplifier, an Er:glass amplifier, or an Er,Yb:glass amplifier. 16.The method of claim 9, wherein the LADAR system is a dual Geiger modeand coherent mode LADAR system, the method further comprising:generating, at a Geiger mode pulse modulator, Geiger mode laser pulses;generating, at a coherent modulator, a coherent LADAR waveform lasersignal at the first wavelength; and separating, at a passivebeamsplitter, the laser pulses at the second wavelength from thecontinuous wave laser signal at the second wavelength.
 17. A Geiger modelaser radar (LADAR) system, the Geiger mode LADAR system comprising: aGeiger mode laser transmitter configured to emit laser pulses at a firstwavelength; a non-linear converter configured to convert the laserpulses to a second wavelength prior to spectral filtering of amplifiedspontaneous emission (ASE) that is emitted from the laser transmitter ina spectrum concentrated around the first wavelength; and a spectralfilter disposed in a transmit path of the Geiger mode LADAR system, thespectral filter configured to substantially filter the ASE and allow thelaser pulses at the second wavelength to pass.
 18. The Geiger mode LADARsystem of claim 17, wherein the second wavelength is outside of at leasta substantial portion of the ASE spectrum.
 19. The Geiger mode LADARsystem of claim 17, wherein the non-linear converter employs anintensity-dependent conversion process that converts the laser pulses tothe second wavelength but does not substantially affect the ASE.
 20. TheGeiger mode LADAR system of claim 17, further comprising: a secondspectral filter disposed in a receive path of the Geiger mode LADARsystem, the second spectral filter configured to filter at least some ofthe ASE and allow the laser pulses at the second wavelength to pass.